The electrical insulation in large motors or generators plays a vital role. Breakdown of this insulation can result in major down time for the machine; in fact, industry surveys show that insulation system failure is one of the major causes of motor breakdowns and generator outages.
In recent years utilities have reactivated their interest in motor and generator insulation conditions because the deferral of new generating capacity places greater emphasis on the reliable operation of older power plants. To effectively maintain motors and generators, utilities need periodic information on the insulation conditions. In addition, continuous condition assessments may be necessary to avoid in-service failures.
The measure of temperature of the stator of a rotating electrical machine presents little difficulty. The measure of temperature in rotors requires the ability to transfer data from the rotating portion of the machine to the stationary portion of the machine.
Slip rings have been used in the past for transfer of data; however, these have not been satisfactory because of wear and electrical interference.
Other systems have employed transformers to transform signals whose frequency is dependent on temperature from the rotor to the stator of the machine. Power has been supplied to the circuits mounted in the rotor by induction. In U.S. Pat. No. 3,824,857 there is described a temperature measuring system in which the output of the thermocouple is used to control the frequency of a voltage-to-frequency converter. In one embodiment, the signal is transformer coupled to a stationary portion of the machine. In another, it is coupled by an FM transmitter and receiver.
None of the prior art systems provide a plug including a temperature sensor and transmitter which can be easily installed in the rotor of a generator or motor.